The present invention relates to a ferroelectric memory device having memory cells each of which stores therein binarized data as a polarization state of a ferroelectric layer, and a manufacturing method thereof.
An FeRAM (Ferroelectric Random Access Memory) has been known as a so-called ferroelectric memory.
As one example of the ferroelectric memory, there has been known a configuration wherein an upper electrode of a capacitor including a ferroelectric layer and a W (tungsten)-plug connected to its corresponding elemental device are connected by a local wiring (refer to a non-patent document 1 (IEDM Digest of Technical papers. 1997, pp 613-616 (refer particularly to FIG. 1)).
There has also been known a configuration wherein a lower electrode of a capacitor including a ferroelectric layer and a W (tungsten)-plug connected to its corresponding elemental device are directly connected (refer to a non-patent document 2 (IEDM Digest of Technical papers. 2002, pp 539-542 (refer particularly to FIG. 3)).
A ferroelectric layer included in the FeRAM is formed of an oxygen compound material like so-called SBT (SrBi2Ta2O9). The ferroelectric layer will cause a reduction reaction due to moisture (H2O) inevitably mixed in, for example, a CVD film formed around the ferroelectric layer, and hydrogen (H2) derived from the moisture or hydrogen evolved upon formation of each embedded contact (plug). Due to this reduction reaction, the polarization characteristic of the ferroelectric layer is degraded.
After the formation of the capacitor including the ferroelectric layer, heat treatment is generally done under an oxygen atmosphere for the purpose of recovering damage of the ferroelectric layer. At this time, however, there is a case where when the surface of the plug is being exposed, the plug is oxidized, so that faulty electrical continuity occurs.
With the objective of reducing damage of a ferroelectric film (layer) due to the spreading of hydrogen or moisture through contacts each extending to an upper electrode of a so-called ferroelectric memory, there has been known a method for manufacturing a semiconductor device wherein conductive hydrogen barrier films are formed after the formation of such contacts (refer to a patent document 1 (Japanese Unexamined Patent Publication No. 2002-252336)).
With a view toward to suppressing and preventing degradation of the characteristic of a memory cell capacitor due to hydrogen and a reductive atmosphere, there have been disclosed a semiconductor memory device wherein a first hydrogen barrier film for covering the side above the memory cell capacitor and a second hydrogen barrier film for covering the side below the memory cell capacitor are provided, and a method for manufacturing the same (refer to a patent document 2 (Japanese Unexamined Patent Publication No. 2003-68987)).
Further, with a view toward preventing faulty electrical continuity due to the oxidation of each plug, there has been known a configuration wherein an wiring on an upper electrode of a semiconductor device is connected to its corresponding plug via a conductor formed in the same process as one for a lower electrode of a ferroelectric capacitive element (refer to a patent document 3 (Japanese Unexamined Patent Publication No. 2001-250922)).
There may be cases in which even though each hydrogen barrier film for covering the upper or lower side of the capacitor including the ferroelectric layer is formed as disclosed in the patent documents 1 and 2, for example, hydrogen reaches the ferroelectric layer through the corresponding local wiring connected to the upper electrode of the capacitor, thus causing degradation of the ferroelectric layer. Since no large current flows in the local wirings in the ferroelectric memory having such local wirings as described in the non-patent document 1, TiN (titanium nitride) is generally applied as a wiring material.
The present TiN wiring is capable of absorbing or sucking and holding hydrogen of a certain degree of amount. When, however, the hydrogen exceeds its allowable amount, the TiN wiring discharges the absorbed hydrogen. That is, hydrogen existing inevitably outside the capacitor, which evolves upon the formation of each plug, for example, is brought to the ferroelectric capacitor through the TiN wiring even though the upper and lower sides of the capacitor are covered with the corresponding hydrogen barrier film, where a portion exposed outside the hydrogen barrier film exists in the TiN wiring. As a result, the ferroelectric layer is degraded.